trans

128 ENERGY METABOLISM – ANAEROBIC PROTOZOA
other type of metabolic organization does not
imply a monophyletic origin of all Type I or all
Type II organisms. The data strongly suggest
that both organizational types arose independently
in different evolutionary lineages several
times.
STEPS OF
AMITOCHONDRIATE CORE
METABOLISM
As stressed above, the central core of amitochondriate
energy metabolism is the classical
Embden–Meyerhof–Parnas glycolytic pathway.
The description of this process will be divided
into four parts: (a) uptake of exogenous carbohydrate
and mobilization of endogenous carbohydrate
reserves (Figure 7.1); (b) conversion
of hexoses into triosephosphates (Figure 7.1);
(c) conversion of triosephosphates into phosphoenolpyruvate
(Figure 7.1); and (d) further
conversions of phosphoenolpyruvate (PEP)
(Figures 7.2 and 7.3).
Uptake of exogenous carbohydrate
and mobilization of endogenous
carbohydrate reserves (Figure 7.1)
The organisms discussed all utilize glucose
taken up by active transport [reaction b].
Tr. foetus stands alone in also being able to utilize
fructose. They can also utilize to different
extents maltose (a dimer of glucose) and
glucose polymers that are hydrolyzed extracellularly.
A cell-surface-bound extracellular
-glucosidase splits maltose to glucose in
T. vaginalis [reaction a], in a process similar to
the ‘membrane digestion’ described for intestinal
cells and platyhelminths.
High levels of glycogen are the intracellular
carbohydrate reserves in all four species. The
mobilization of these reserves has not been
studied in detail, but it probably occurs by
phosphorolysis forming glucose-1-phosphate
[reaction d], subsequently converted to glucose-
6-phosphate by phosphoglucomutase [reaction
e]. Glycogen phosphorylase has been
detected in E. histolytica and Tr. foetus, and
phosphoglucomutase in E. histolytica.
Conversion of hexoses into
triosephosphates (Figure 7.1)
The entry of exogenous glucose into the glycolytic
pathway is through phosphorylation
by an ATP-linked kinase (glucokinase), forming
glucose-6-phosphate [reaction 1]. Glucose-
6-phosphate derived from the intracellular
glycogen reserves enters the pathway at this
level. Glucose-6-phosphate is converted to
fructose-6-phosphate by glucosephosphate
isomerase [reaction 2]. Tr. foetus also contains
a fructokinase [reaction c] in agreement with
the utilization of fructose by this species.
These steps do not differ in their mechanism
from the processes in other eukaryotes, save
in the narrow substrate specificity and regulation
of glucokinase.
Phosphorylation of fructose 6-phosphate to
fructose-1,6-bisphosphate is catalyzed by PP i -
phosphofructokinase (PFK) in a reversible
reaction using PP i as phosphoryl donor [reaction
3]. This enzyme is present in diverse
organisms, including mitochondriate ones,
but is less ubiquitous than the broadly distributed
ATP-PFK, which catalyses an irreversible
process. The reversibility of PP i -PFK indicates
that this step is not regulated. Fructose bisphosphate
is subsequently split into dihydroxyacetone
phosphate and glyceraldehyde
3-phosphate by type II (metal dependent)
fructose-1,6-bisphosphate aldolase in an aldol
cleavage reaction [reaction 4].
BIOCHEMISTRY AND CELL BIOLOGY: PROTOZOA